Serotonine levels in the brain have been linked with mood, alertness, vigilance, sleep onset and quality, anxiolytic effects, depression, affective reaction control, appetite and sexual behavior. Many publications exist in which changes in brain serotonin levels are correlated with the availability of the natural amino acid L-tryptophan (Trp or W). Because of this correlation, methods to increase plasma tryptophan levels have received a lot of attention. Tryptophan quantities of around 1 gram/day per individual have been reported to yield clinically significant effects (Markus et al., Am. J. Clin. Nutr 2005; 81, 1026-1033). One method to increase plasma tryptophan levels involves the consumption of protein preparations enriched in the whey protein alpha-lactalbumin. Alpha-lactalbumin preparations are readily available and have a relatively high tryptophan concentration. However, approaches in which the alpha-lactalbumine is provided as such, see for example DE 4130284 and JP 2279700, do not take into account that the main determinant of brain tryptophan and serotonin levels is not plasma tryptophan concentration alone, but the so-called Trp/LNAA ratio (Fernstrom and Wurtman. Science 1971, 173, 149-152). This Trp/LNAA ratio represents the molar ratio of tryptophan relative to the levels of Large Neutral Amino Acids (LNAA: i.e. the sum of tyrosine, phenylalanine, leucine, isoleucine and valine) in plasma. These LNAA compete with tryptophan for uptake into the brain, presumably because the same transport mechanism across the blood-brain barrier is used. Therefore, the most effective way of increasing brain tryptophan concentrations is to supply preparations with a high Trp/LNAA ratio. A number of publications a.o. WO 02/46210, refer to the preparation of peptide fractions from alpha-lactalbumin having improved Trp/LNAA ratio's.
The use of free tryptophan, i.e. the free amino acid, would provide the simplest and cheapest approach to provide preparations with a high Trp/LNAA ratio. However, in many countries legislation exists that tightly regulates the supply of free tryptophan. The maximal allowable free tryptophan levels in its various application forms vary per country. To supply additional dietary tryptophan in a more natural way, more recent approaches aim at providing tryptophan rich proteins. As mentioned, alpha-lactalbumin as well as its hydrolysates have gained popularity as a safe option to enhance plasma tryptophan levels. However, the use of alpha-lactalbumin as a starting point for tryptophan-rich preparations, comes with disadvantages in terms of maximal Trp/LNAA ratios and costs. Alpha-lactalbumin and beta-lactoglobulin form the major protein constituents of whey. Because on an industrial scale a complete separation of alpha-lactalbumin and beta-lactoglobulin is difficult, the implication is that cost effective alpha-lactalbumin preparations will contain beta-lactoglobulin as well. Whereas alpha-lactalbumin has a molar tryptophan content of 5.3%, the tryptophan content of beta-lactoglobulin is only 2%. Whereas alpha-lactalbumine has a molar Trp/LNAA ratio of 0.11, beta-lactoglobulin has a molar Trp/LNAA ratio of not more than 0.04. So obviously any contamination of the alpha-lactalbumin preparation with beta-lactoglobulin, will dramatically lower the Trp/LNAA ratio of the final product.
In view of the broad interest in preparations that can modulate brain serotonine levels, there is a need for improved production methods for protein and peptide preparations having a high Trp/LNAA ratio that are broadly applicable in various food and neutraceutical products.